Apparatus for coating a lightguide fiber

ABSTRACT

A lightguide fiber (21) which has been drawn from a preform in a furnace (26) and which has a relatively high temperature is cooled prior to its movement through an apparatus (41) which applies a coating to its outer surface. The fiber is cooled by moving it through a liquid material (42) in a reservoir (40) which is interposed between the furnace and the coating apparatus. The liquid material in the reservoir (40) is the same as the coating material contained in the apparatus that is used subsequently to coat the fiber.

This is a division of application Ser. No. 509,683 filed June 30, 1983.

TECHNICAL FIELD

This invention relates to apparatus for coating elongated material. Moreparticularly, it relates to apparatus for applying a layer of a coatingmaterial concentrically about a lightguide fiber which is drawn from apreform.

BACKGROUND OF THE INVENTION

The successful implementation of a lightwave communication systemrequires the manufacture of high quality lightguide fibers havingmechanical properties sufficient to withstand stresses to which they aresubjected. Each fiber which is made of glass must be capable over itsentire length of withstanding stresses that it will encounter duringinstallation and service. The importance of fiber strength becomesapparent when one considers that a single fiber failure will result inthe loss of several hundred transmission circuits. The failure oflightguide fibers in tension is commonly associated with surface flawswhich cause stress concentrations and lower the tensile strength fromthat of the pristine unflawed glass.

The potential strength of a lightguide fiber is realized only if it isprotected with a relatively thin layer of a suitable coating materialsoon after it has been formed, such as by drawing it from a preform.This coating which has a thickness of about 0.005 cm serves to preventairborne particles from impinging upon and adhering to the surface ofthe fiber which would serve to weaken it. Also, the coating shields thefiber from surface abrasion, which could be inflicted by subsequentmanufacturing processes and handling during installation, providesprotection from corrosive environments, and spaces the fibers in cablestructures.

Important properties relating to the coating are its concentricity, andits thickness. An off-centered fiber in the coating may not adequatelyprotect the fiber surface which could have an adverse effect on fiberstrength and microbending loss. The coating must be thick enough toadequately cover and protect the surface of the fiber, but not so thickthat it impairs subsequent manufacturing operations and/orconnectorization.

In one process, the coating is applied by advancing the lightguide fiberthrough a reservoir of an open cup applicator containing a liquidpolymeric material. Typically, the fiber enters the coating materialthrough a free surface, and exits through a relatively small die orificeat the bottom of the reservoir.

Uniform wetting of the fiber during the coating process is largelyaffected by the behavior of an entrance meniscus which exists where thefiber is advanced through the free surface of the coating material inthe reservoir. As is well known, the wetting characteristics of twomaterials such as a liquid coating material and glass, depend on surfacetension and chemical bonds which are developed between the twomaterials.

The wetting characteristics are affected by a pumping of air into themeniscus. The fiber pulls a considerable amount of air into the coatingmaterial as it enters the free surface of the coating applicator. Theentrance meniscus is drawn down with the moving fiber, instead of risingas it does under static conditions.

As the line speed is increased, the meniscus extends downwardly anddevelops into a long, thin column of air which surrounds the fiber andwhich is confined by surface tension in the coating material. Themeniscus becomes unstable, oscillating between a fully developed statewith circulation and a relatively small size with little or nocirculation.

Should the column of air be caused to extend completely through thecoating material to the die orifice, the meniscus collapses and thefiber no longer contacts the coating material. A meniscus may bereformed and the process of collapse repeated. If meniscus collapse hasoccured, the fiber may be coated but there is insufficient wetting toobtain a uniform covering of substantially the entire outer surface ofthe fiber. As a result, the strength and size of the fiber are adverselyaffected and the likelihood for damage and transmission losses isincreased.

Meniscus collapse may be caused as a result of higher line speeds andthe relatively high temperature of the fiber as it emerges from adrawing furnace. As the fiber is moved from the furnace into a reservoirof coating material, the air surrounding the fiber expands and preventscontact of the coating material with portions of the fiber. If the fiberis cooled sufficiently prior to its passage through the coatingreservoir, meniscus collapse is avoided.

The prior art has addressed this problem. A solution is to space thecoating applicator a sufficient distance from the drawing furnace sothat the fiber is cooled by ambient air. Then when the fiber enters thecoating applicator, it is sufficiently cool and meniscus collapse isavoided. In some instances however, it may not be feasible to increasethe distance between the furnace and the coating applicator. Physicalrestrictions of the building in which fiber drawing apparatus is locatedmay preclude the use of such a solution. Possibly, sufficient coolingtime could be realized with the present arrangement by reducing linespeed. Because meniscus collapse is affected by line speed as well as bythe temperature of the fiber, a reduction in line speed is particularlyhelpful in decreasing the probability of meniscus collapse. However,from an economic standpoint, a decrease in line speed is not a desirablesolution.

What is needed and what is not provided by the prior art is a coatingarrangement in which the drawn fiber is provided with a coating havingsufficient thickness without requiring undue space between the drawingfurnace and the coating cup.

SUMMARY OF THE INVENTION

The foregoing problems have been overcome by the apparatus of thisinvention. In order to uniformly coat a lightguide fiber, the fiberwhich may be drawn from a glass preform in a furnace is advanced along apath of travel. After it leaves the furnace, the fiber is moved througha first reservoir of a liquid material. The first reservoir isinterposed between the furnace and a second reservoir of a liquidcoating material which causes a coating of the material to be applied tothe fiber. The interposed reservoir holds a material which is identicalto that which is applied subsequently as a coating to the fiber. Also,the coating material in the first reservoir is maintained at atemperature which is substantially less than the temperature of thefiber as it is formed. The temperature of the material in the interposedreservoir is sufficiently low to cool the fiber so that when it isadvanced through the second reservoir, the coating material thereincontacts and covers substantially the entire surface of the fiber.

In another embodiment, the apparatus of this invention is used to cool afiber preparatory to its being advanced through apparatus which appliesa dual coating to the fiber. The dual coating may comprise an innerlayer of a relatively low modulus curable polymeric material and anouter layer of a relatively high modulus curable polymeric material.Prior to the application of the dual coating, the fiber is cooled bymoving it through a reservoir which contains the same liquid materialwhich is used to form the inner layer on the fiber. In an alternativeembodiment, the cooling is supplemented by providing an entry die to thecoating apparatus and by cooling that die to reduce the temperature ofthe lightguide fiber and its surrounding coating material.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be more readily understoodfrom the following detailed description of specific embodiments thereofwhen read in conjunction with the accompanying drawings, in which:

FIG. 1 is an overall perspective view of a portion of a line on whichlightguide fiber is drawn from a preform and covered with a coating of apolymeric material;

FIG. 2 is an end cross-sectional view of a coated lightguide fiber;

FIG. 3 is an elevational view in section of facilities for cooling thedrawn lightguide fiber;

FIG. 4 is an elevational view in section of an applicator for providinga coating about the drawn, cooled lightguide fiber;

FIG. 5 is an end cross-sectional view of a dual coated lightguide fiber;and

FIG. 6 is an elevational view of a portion of a lightguide fiber drawingline which shows facilities for applying dual coatings to the drawnfiber.

DETAILED DESCRIPTION

Referring now to FIG. 1 there is shown an apparatus which is designatedgenerally by numeral 20 and which is used to draw a lightguide fiber 21having a diameter of about 0.012 cm from a specially preparedcylindrical preform 22 and to provide the fiber with a protectivecoating 23 (see FIG. 2). The preform is fed into and through a furnace26 where it is heated to allow the fiber 21 to be drawn therefrom. Theapparatus 20 also includes a device 28 for measuring the drawn fiber.After the fiber diameter is measured, the coating is applied by anapparatus 30. Then, after the coated fiber 21 passes through a centeringgauge 32, a device 34 for treating the coating and a device 36 formeasuring the outer diameter of the coated fiber, it is moved through acapstan 38 and is taken up.

The temperature inside the furnace 26 is about 2000° C. The temperatureof the drawn fiber upon its exit from the furnace is about 1700° C. Aswas mentioned hereinbefore, the temperature of the fiber, if not reducedprior to entry into a coating applicator, causes meniscus collapse whichresults in a non-uniform coating.

The apparatus 30 overcomes this problem of meniscus collapse caused bythe temperature of the fiber 21 without sacrificing line speed. It doesso in one portion thereof by causing the temperature of the moving fiber21 to be reduced substantially prior to its entry into another portionwhich applies a uniform coating thereto. Also, the apparatus 30 does notrequire substantial additional space and existing lines may beretrofitted with them. After the fiber has been cooled by a portion ofthe apparatus 30, it is moved through another portion which applies auniform coating.

In a preferred embodiment, the apparatus 30 includes a cooling device39. The cooling device 39 is the portion of the apparatus 30 which iseffective to decrease substantially the temperature of the newly formedfiber 21. The device 39 includes a reservoir which is designatedgenerally by numeral 40 (see FIGS. 1 and 3) and which is interposedbetween a coating applicator 41 for applying a single coating to thefiber 21 and the furnace 26.

The reservoir 40 is used to hold a liquid coating material 42 which isthe same as the coating material in the coating applicator 41 andthrough which the lightguide fiber 21 is moved. An example of a coatingmaterial is a UV curable polymeric material. The reservoir 40 includesan inner vessel 43 which includes a flange 44 supported on an upper endof a wall 45 of the reservoir 40. The joint between the flange 44 andthe upper end of the wall 45 is sealed with an O-ring 46.

The vessel 43 has a conically shaped intermediate portion 48 whichterminates in a cylindrical portion 50. The cylindrical portion 50 and aportion of the conically shaped portion 48 include a vertically disposedpassageway 52. Typically, the passageway 52 is slightly larger than theoutside diameter of the lightguide fiber 21 being moved therethrough. Aflanged lower end of the cylindrical portion 50 of the vessel 43 issealed from a lower portion of the reservoir 40 by an O-ring 53.

From a lower end of the cylindrical portion 50 depends anothercylindrical portion of a smaller diameter and designated 54. Thepassageway 52 through the conically shaped portion 48 and thecylindrically shaped portion 50 also extends through the portion 54 andterminates in an exit orifice 56 of a die 57. The die orifice 56communicates with a chamber 58 formed in a lower end of a portion 60having a flexible tip 62 depending therefrom. The tip 62 provides asecond die 64 having an exit orifice 66.

As can be seen in FIG. 3, the wall of the reservoir 40 includes an inlet71 through which chilled water is fed to fill a chamber 72 between thewalls of the reservoir and the inner vessel 43. Chilled water at atemperature of about 25° C. is introduced through the inlet 71 and intothe chamber 72 with an exit port 73 being provided to permit the waterto be recirculated.

The coating material 42 extends from a free surface in the inner vessel43 through the passageway 52 into the chamber 58. Inasmuch as thecoating material 42 is moved along with the fiber 21 and through theelongated passageway 52 through the die 57 and into the chamber 58, aportion of the material is constantly being depleted from the reservoir.Accordingly, this material is replenished through a conduit 76 whichcommunicates with a supply (not shown) of the coating material. TheO-ring 53 prevents commingling of the cooling medium and the coatingmaterial 42.

The temperature of the liquid plastic material in the apparatus 40 ismaintained within a range to ensure that sufficient heat energy isextracted from the moving fiber 21. This temperature is affected by linespeed. For higher speeds, a lower temperature is used. For a preferredembodiment in which the line speed is in the range of 3 to 5meters/second, the temperature of the coating material 42 in thereservoir 40 should not exceed a range of about 38°-43° C. andpreferably should be ambient.

This invention overcomes the problem of meniscus collapse and incompletecoating. Coating instability occurs in the cooling device 39 but by thetime the fiber has reached the coating applicator 41 it has cooledsufficiently to avoid instabilities. Meniscus collapse is avoided in thecoating applicator. The hot air which surrounded the fiber 21 in thecooling reservoir 40 has been stripped away. A meniscus which is formedand which is helpful to the coating of the fiber is stable. As a result,the coating which is applied by the applicator 41 is substantiallyuniform over the entire surface of the fiber 21.

The applicator 41 is shown in detail in FIG. 4. It is disclosed andclaimed in application Ser. No. 343,134 which was filed on Jan. 27, 1982in the names of C. J. Aloisio et al, now U.S. Pat. No. 4,409,263 whichissued Oct. 11, 1983, and which is incorporated by reference hereintoapplication Ser. No. 343,134 issued as U.S. Pat. No. 4,409,263 on Oct.11, 1983.

As can be seen in FIG. 4, the applicator 41 for coating the drawnlightguide fiber 21 includes a housing 80 to provide a reservoir of thecoating material 42 which is recirculated and replenished. The housing80 includes a depending portion 82 and an insert 84. A lower end of theinsert 84 includes a die 85, which is designated as the first die. Thehousing 80 includes a cylindrical wall section 91 which is joined to adiverging wall portion 92. The portion 92 provides a cavity 94 having atruncated conical shape which communicates with a lower portion 96having a generally constant diameter.

The insert 84 includes an upper portion 101 having a flange 102 which issupported on the wall section 91. A constant diameter section 103 isreceived within the constant diameter section 91 of the housing and isjoined to a conically shaped portion 104 having a wall thickness whichincreases in a downward direction. A lower end of the conically shapedportion 104 is connected to a constant diameter portion 107 whichextends into a cylindrical cavity 108 of the depending portion 82.

The coating material 42 extends from a supply conduit 109 through achamber 110 below the insert 84 and upwardly to a reservoir 111 formedby the insert. The coating material in the reservoir 111 has a freesurface 112. An overflow opening 113 for the coating material isprovided. The coating material 42 in the reservoir 111 has a temperaturein the range of about 30°-40° C.

The chamber 110 communicates with the reservoir 111 through an elongatedpassageway 114 which extends through the constant diameter portion 107.The elongated passageway 114 has a diameter of about 0.30 cm and alength of about 4 cm. A land 115 of the die 85, which has a length ofabout 0.6 cm, is joined to the elongated passageway 114 through atapered section 116. The land 115 includes an exit orifice 117 (see FIG.4) which has a diameter of about 0.076 cm and which opens to the chamber110. The cavity 108 and the constant diameter portion 107 provide a flowpassage 126 of constant cross-section from the vicinity of the supplyconduit 109 to the vicinity of the exit orifice 117 of the die 85.

The elongated passageway 114 and other portions of the die cavity of thefirst die may be considered as a "lubricated core tube" through whichthe fiber 21 is advanced. It is lubricated because unlike in the closefitting core tube of a conventional plastic extruder for copper wire,the coating material is flowing through it. Because of its length, thepassageway 114 supplements the cooling device 39 in preventing meniscuscollapse.

A conically shaped second die 131 is connected to the depending portion82. The second die 131 is made of a semi-rigid material such asfluorosilicone rubber, for example, and includes a cavity 132 having atruncated conical shape. The cavity 132 communicates through a land 134with an exit orifice 136. The land 134 has a diameter of about 0.025 cmwhich is about the diameter of the coated fiber and a length of about0.10 cm.

The coating material 42 is directed through the conduit 109 into thechamber 110 at sufficient positive pressure to cause a volumetric flowof the coating material upwardly through the first die 85 into thereservoir 111. A suitable pressure is in the range of about 40 newtonsper square centimeter. As the fiber 21 is advanced, some of the coatingmaterial 42 is pulled downwardly through the first die 85 into thechamber 110.

The fiber 21 is advanced along a path of travel 138 at a velocity whichcauses air to become entrained in the in the coating material 42. Forthe coating applicator 41, there are two pressure gradients whichcontribute to the removal of bubbles from the fiber 21. One occursbetween portions within the first die 85; the other between the chamber110 and a point just prior to the orifice 136 of the second die 131. Theadvance of the fiber 21 causes a pressure gradient to be establishedalong its path of travel between the free surface 112 and the first die85 with the gradient being a maximum at the exit orifice 117. Thepressure within the coating material 42 decreases abruptly from the exitorifice 117 of the first die 85 to an exit 140 of the cylindricalpassageway 114, then gradually to an entrance 141 thereof and stillfurther to the free surface 112. Following the exit orifice 117 of thefirst die 85, the pressure increase to a point in the vicinity of thesecond die 131. Afterwards, the pressure drops to zero at the exitorifice 136 of the second die 131.

The flowing of the pressurized coating material 42 into the chamber 110enhances the first pressure gradient between portions of the first die85. The enhanced pressure gradient between portions of the first die 85cooperates with the volumetric flow of coating material upwardly toremove the air packets and any resulting bubbles from the advancingfiber 21. This causes air packets which have been carried along with thefiber 21 to bulge into a bubble shape and attach to recirculatingstreamlines. The gradient between a point within the chamber 110 and apoint prior to the exit orifice 136 of the second die 131 is effectiveto strip any bubbles that may be in the chamber 110 between the dies 85and 131.

Bubbles which are removed by the first die 85 and released into thestreamlines are carried upwardly into the reservoir 111. The bubbleswhich are returned upwardly into the reservoir 111 coalesce, break apartor flow out of the applicator 41 through the opening 113. As a result,any coating material 42 in the chamber 110 is substantially bubble-free.

There is some tendency for the fiber 21 to be miscentered as it is movedthrough the first die 85. However, the pressure in the 0.076 cm firstdie 85 and the incoming coating material cooperate and act as adampening agent to stabilize the fiber 21. Because of locating forcescaused by the coating material in the first die 85, the vibrationscaused by the coalesced large bubbles in the reservoir 111 are not seenin the final die 131. The net result is that the coating issubstantially concentrically disposed about the fiber 21.

The methods and apparatus of this invention also are useful in a processin which two coatings are applied to the lightguide fiber 21 to providethe product shown in FIG. 5 and designated 150. Dual coated lightguidefibers are used to obtain design flexibility and improved performance.Typically, an inner or primary coating layer 151 that comprises alow-modulus material is applied over the fiber 21. Such a materialreduces microbending losses associated with the cabling, installation,or environmental changes during the service life of the fiber. An outeror secondary coating layer 152 is applied to the primary layer. Theouter layer 152 is usually a higher modulus material to provide abrasionresistance for the fiber 21 and the primary coating layer 151. See anarticle authored by M. Sato et al, and entitled "Double Layer SiliconeCoating With Double Cone Nozzle In-Line with Optical Fiber Drawing,"which appeared in the Proceedings of the Fifth European Conference onOptical Communications, pages 5.6-1 to 5.6-4 (Amsterdam, 1979).

Although some coating materials which are used to form the inner layer151 are not affected by the temperature of the drawn fiber and acontinuous coating is achieved, others are and a sporadic coatingensues. Accordingly, when applying such a coating material which is soaffected to a drawn fiber, it becomes advantageous first to cool thefiber 21 and then to move it through apparatus which applies the twocoating layers. As is shown in FIG. 6, an apparatus designated generallyby the numeral 200 for applying multiple coatings to the fiber 21includes an upper housing 202 having a threaded portion 204 dependingtherefrom. The upper housing 202 includes a stepped cavity 206 whichcommunicates with an exit passage 208 through a tapered bore 210.

Mounted within the cavity 206 of the upper housing 202 is a core tube212. The core tube 212 has an enlarged end portion 214 which issupported on a step 216 of the housing 202, an intermediate portion 217which is supported on a step 218 and an elongated portion 220. A tip 222of the core tube is beveled and terminates at the beginning of thetapered bore 210. The core tube 212 also includes a passageway 224.

The enlarged and intermediate portions 214 and 217, respectively, have acavity 226 formed therein for receiving an entry die 228 in a press fit.The entry die 228 includes a tapered entrance 231 which communicatesthrough a tapered bore 233 and a land 236 with the passageway 224.

An upper end 238 of the upper housing 202 is threaded to receive a cap241. The cap 241 includes an entrance 242 having a first frustoconicalportion 243 and a second frustoconical portion 244. The second portion244 communicates with the entrance 231 of the entry die 228. The cap 241holds the assembly together by maintaining the enlarged end 214 inengagement with the step 216. An O-ring 246 provides a seal between theenlarged end 214 and the step 216.

The threaded portion 204 of the upper housing 202 is turned into acavity 251 of a lower housing portion 253 with an O-ring 254 forming aseal therebetween. The lower housing 253 includes a stepped cavity 256for holding a die insert 258. The die insert 258 is spaced slightly froma lower end of the threaded portion 204 and includes a passageway 261therethrough. The passageway 261 includes an upper portion 262, atapered portion 264 and a land 266 which terminates in an exit orifice268 of a first die 269.

The lower housing 253 is fitted with an end cap 271. An O-ring 273provides a seal between abutting surfaces of the lower housing 253 andthe end cap 271. As can be seen in FIG. 6, the end cap 271 includes acavity 274 into which extends a tapered portion 276 of the die insert258. The cavity 274 communicates through a land 278 with an exit orifice280 of a second die 281.

The apparatus 200 is used to apply the inner and outer coatings to alightguide fiber 21 moved therethrough. A first liquid coating materialwhich also fills the cooling device 39 and which is destined to form theinner layer 151 is supplied to a channel 282, a portion 286 of which isfluted. The first material which for coating one kind of fiber may below modulus UV curable polymeric material fills the cavity 210 and thepassageway 261. Further, it flows upwardly into the elongated passageway224 and is maintained at a level designated 288 within the set pointentry die 228. A second liquid coating material, which is used toprovide the outer layer 152 for the fiber 21 is supplied to the cavity274. The second coating material may be a higher modulus UV curablepolymeric material.

The coatings which are applied to the lightguide fiber 21 should bebubble-free. Apparatus such as that disclosed in application Ser. No.454,159 filed on Dec. 29, 1982 now, U.S. Pat. No. 4,474,830, in the nameof C. R. Taylor provides such dual coatings. In the apparatus 200, thisis accomplished by maintaining the first coating material at thepredetermined level designated 288 in the entry die 228. Bubble freecoatings also are achieved by maintaining the virgin coating material ina supply tank (not shown) free of bubbles.

The pressure of the first coating material is in the range of 30 to 70psi. It is a function of the line speed and of the exit orifice of theentry die 228. As the fiber 21 is advanced through the apparatus 200,heat energy which has not been totally extracted from the fiber in theapparatus 30 causes the temperature of the first coating material to beincreased. If this change is not addressed, it would cause the level 288in the entry die 228 to rise and may result in bubbles in the coating.Accordingly, monitors (not shown) are used to detect the level of thefirst coating material above the elongated passageway 224. As thetemperature of the first coating material increases and the level tendsto rise, the detectors are effective to cause the pressure to bereduced. The results in the free surface of the coating material beingmaintained at the predetermnined level 288.

The die sizes are also important to the successful coating ofsubstantially the entire surface of the moving fiber 21. The first die269 should have an orifice diameter in the range of 0.023 to 0.025 cmand the entry die 228, an orifice of about 0.076 cm. The second die 281should have an orifice size in the range of 0.037 to b 0.038 cm.

The relatively long passageway 224 is used to aid in developing the flowof the first coating material. The first coating material enters thechannel 282 at about the fluted portion 286 of the core tube, isdirected downwardly into the passage 208 and cavity 261 and upwardlyinto the elongated passageway 224. This provides time for the flow todevelop into a balanced flow before contacting the fiber 21.

The size of the meniscus at the free surface in the entry die 228 isdependent on the geometry of the surrounding structure. The entry die228 is helpful in preventing the formation of an overly large meniscuswhich would increase the probability of collapse. Its relatively smallsize prevents the formation of an overly large meniscus therebydecreasing the probability of meniscus collapse. Of course, if the drawnfiber had been cooled by the device 39, then notwithstanding thegeometry of the die 228, the pressure could be high enough to trigger ameniscus collapse.

In an alternative embodiment, the entry die 228 may be cooled byintroducing chilled water into passages 290--290 (see FIG. 6). Thissupplements the cooling device 39 and further decreases the temperatureof the fiber 21 by conductive cooling.

It is to be understood that the above-described arrangement are simplyillustrative of the invention. Other arrangements may be devised bythose skilled in the art which will embody the principles of theinvention and fall within the spirit and scope thereof.

What is claimed is:
 1. An apparatus for uniformly coating a lightguidefiber, said apparatus including:means for providing a lightguide fiberwhich has a relatively high temperature; means for moving the fiberalong a path of travel; coating means disposed along the path of travelfor applying a liquid coating material to the lightguide fiber; andcooling means interposed between said means for providing a fiber andsaid coating means for reducing substantially the temperature of thelightguide fiber, said cooling means comprising;a housing having adepending portion, said depending portion including a die and apassageway which terminates in said die; an inner vessel which ismounted in said housing and which includes an upper portion and a lowerportion which terminates in an orifice of a die and which extends intosaid passageway of said depending portion of said housing, said innervessel including an elongated passageway which extends from said die ofsaid inner vessel to a cavity formed in said upper portion, and saidinner vessel cooperating with said housing to form a chamber which iscapable of being filled with a cooling medium; means for supplying acooling medium to said chamber; and means for providing to said cavityof said inner vessel a liquid material which is the same as the coatingmaterial that is applied by said coating means and which has atemperature that does not exceed a predetermined value.
 2. The apparatusof claim 1, wherein said elongated passageway of said inner vessel has adiameter which is larger than that of said die of said inner vessel andwhich communicates with said die orifice through a tapered portion. 3.The apparatus of claim 1, wherein said die of said housing communicateswith said die orifice of said inner vessel through a tapered portion. 4.The apparatus of claim 1, wherein said coating means includes means forapplying two concentric coatings to the lightguide fiber, the twoconcentric coatings comprising an inner layer of a first polymericmaterial and an outer layer of a second polymeric material.
 5. Theapparatus of claim 4, wherein said coating means also comprises an entrydie into which extends the first coating material, said entry die beingmaintained at a temperature which is effective to cool the fiber beingadvanced through the first coating material.
 6. The apparatus of claim4, wherein said coating means also includes an entry die and means forcausing the first coating material to be maintained at a pressure whichis in a predetermined range.
 7. The apparatus of claim 6, wherein saidmeans for causing the first coating material to have a pressure in apredetermined range causes the first coating material to have a freesurface which is maintained at a predetermined level in said entry die.8. The apparatus of claim 4, wherein said means for applying twoconcentric coatings to the lightguide fiber includes:an upper housingwhich includes a stepped cavity having an elongated portion and anenlarged portion; a core tube which is disposed within said upperhousing, said core tube having an elongated portion which is disposed insaid elongated portion of said stepped cavity and a base which isdisposed in said enlarged portion of said stepped cavity, said core tubeincluding a passageway which extends through said elongated portionthereof and through a portion of said base and which terminates in acavity of said base; an entry die which is disposed in said cavity ofsaid base of said core tube, said entry die having a passageway formedtherethrough; a cap which is attached to an upper end of said upperhousing, said cap having an opening which is aligned with saidpassageway of said entry die; a lower housing which is attached to alower end of said upper housing and which includes a first exit diehaving a passageway which is aligned with said passageway through saidcore tube, said first exit die including a depending portion whichincludes a die orifice; a lower cap which is attached to a lower end ofsaid lower housing and including a second exit die which is aligned withsaid first exit die, said lower cap including a cavity in which isdisposed said depending portion of said first exit die; means forcausing a first liquid coating material to be flowed into a lower end ofsaid core tube, upwardly along said passageway of said core tube and toa predetermined level in said entry die; and means for causing a secondliquid coating material to be flowed into the cavity in which isdisposed said depending portion of said first exit die.
 9. The apparatusof claim 8, wherein an inner surface of said upper housing and an outersurface of said core tube define a flow passage for the first coatingmaterial.
 10. The apparatus of claim 9, wherein a portion of the outersurface of said core tube is fluted.
 11. The apparatus of claim 8,wherein said passageway of said first exit die includes a truncatedconically shaped portion which communicates with the orifice of saidfirst exit die through a land.
 12. The apparatus of claim 8, whereinsaid passageway through said elongated portion of said core tube has aconstant diameter.
 13. The apparatus of claim 12, wherein saidpassageway of said entry die includes an exit orifice which communicatesthrough a land to a portion which converges toward said land, said exitorifice having a diameter which is smaller than that of said passagewayof said core tube.